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The intrinsic viscosity [η] of dendronized linear macromolecules dissolved in thermodynamically good solvents depends both on chain length that is contour length L(c) of the backbone of the polymer and on the persistence length l(p). Investigating a set of samples of identical L(c) but differing in the generation number for which data on lp are available, it becomes possible to establish a generalized viscosity law in the form [η] = K(M/M(p))(α) in which M(p) denotes the molar mass of the objects per l(p), M is the total molar mass, K is a constant, and α is the Mark-Howink exponent, which is found to be 0.55., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The formation of inorganic structures in nature is commonly controlled by biogenic macromolecules. The understanding of mineralization phenomena and the nucleation and growth mechanisms involved is still a challenge in science but also of great industrial interest. This article focuses on the formation and mineralization of two archetypical inorganic materials: zinc oxide and amorphous calcium carbonate (ACC). Zinc oxide is selected as a model compound to investigate the role that polymers play in mineralization. Most of the effort has been devoted to the investigation of the effects of double-hydrophilic block and graft copolymers. Recent work has demonstrated that latex particles synthesized by miniemulsion polymerization, properly functionalized by various chemical groups, have similar effects to conventional block copolymers and are excellently suited for morphology control of ZnO crystals. Latex particles might serve as analogues of natural proteins in biomineralization. The second example presented, ACC, addresses the issue of whether this amorphous phase is an intermediate in the biomineralization of calcite, vaterite, or aragonite. Conditions under which amorphous calcium carbonate can be obtained as nanometer-sized spheres as a consequence of a liquid-liquid phase segregation are presented. Addition of specific block copolymers allows control of the particle size from the micrometer to the submicrometer length scale. The physical properties of novel materials synthesized from concentrated solution and their potential applications as a filler of polymers are also discussed.

Micro- and submicrosized ZnO-polymer hybrid materials were synthesized by precipitating zinc oxide from an aqueous medium in the presence of poly(styrene-acrylic acid) latex nanoparticles, prepared by miniemulsion polymerization. Up to 10 wt % of the latex becomes incorporated into the crystals. Although the long-range order of the inorganic material is essentially not altered by the polymer, studies by photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) show that the latex particles influence the optical and paramagnetic properties of the hybrids, which can be correlated with changes in the defect structure. Typical PL emission spectra showed a narrow UV peak and a defect-related broad band in the green-yellow spectral region. The former emission is attributed to exciton recombination, whereas the latter seems to be related with deep-level donors. Latex acts as a quencher of the visible emission, and compared to pure ZnO, ZnO-latex hybrids show a significantly lower PL intensity in the visible range. A noticeable dynamic behavior of the PL, clearly more remarkable in the presence of latex, was observed, and it is explained in terms of photodesorption of oxygen adsorbed at surface positions. EPR provided additional information about crystal defects and species with unpaired electrons. All EPR spectra showed a single signal at g approximately 1.96, whose intensity and temperature dependence did not correlate with those of the PL visible band. These findings indicate that the green-yellow emission and the EPR signal of our samples have a different physical origins.

An efficient synthesis of 9,9-bis(2-ethylhexyl)fluorene oligomers up to the heptamer is reported, with repetitive Suzuki and Yamamoto coupling reactions employed in the synthesis. The key steps for preparation of the essential intermediates include Pd-catalyzed transformation of aryl bromides to aryl boronic esters (Miyaura reaction) and the application of the much higher reactivity of aryl boronic esters over aryl bromides in the Pd-catalyzed cross-coupling reaction with aryl diazonium salts. Variation of the UV/Vis absorption and photoluminescence characteristics with chain length is reported. Moreover, glass transition and liquid-crystal characteristics of the oligomers are described and compared with those of the polymer.

Hole transport in a polyspirobifluorene homopolymer and a statistical polyspirobifluorene-triarylamine copolymer has been studied in detail employing the charge-generation layer time-of-flight (TOF) technique over a wide range of electric fields and temperatures. Both materials exhibit nondispersive TOF signals after injection of a sheet of charge carriers from a thin (10 nm) perylene-diimide charge-carrier generation layer into a relatively thick (d>1 μm) polymer film. Results were analyzed within the framework of the Gaussian disorder model and the charge transport parameters were extracted for both polymers. The zero-field hole mobility of the spirohomopolymer was found to be on the order of 10-6 cm2/V s, whereas the copolymer showed a considerably lower hole mobility of 6×10-8 cm2/V s. The width of the density of states σ was determined to be 86 meV for the homopolymer and 107 meV for the copolymer. The latter polymer also showed an increased positional disorder due to the statistically incorporated triarylamine units. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) properties of oligofluorenes with 2-ethylhexyl group in 9, 9′ position in solution and as thin films were investigated by time-resolved techniques at both room temperature and 77 K. The fluorescence lifetimes of the oligomers decrease with chain length. The lifetimes τ follow the relation τ=386+808(1/n) (ps) where n is the number of fluorene units in the oligomer. Concentration and laser excitation energy dependences of PL spectra of the oligofluorenes are also given. Phosphorescence was observed for oligofluorenes in the frozen matrix of MTHF at 77 K. The lifetime of phosphorescence increases with increasing molecular length. Similar emission bands were observed for oligofluorenes with a central ketogroup. A lifetime analysis clearly reveals that the “green emission” of the oligomers free of ketogroups results from a phosphorescence with lifetime τ of 3 ms while the green emission from the keto-oligomer is a fluorescence from a charge transfer π-π* level of τ=8 ns. [ABSTRACT FROM AUTHOR]

This review covers the most recent developments using the Polymer Reference Interaction Site Model (PRISM) integral equation theory to study polymer melts and blends. Comparisons to computer simulations are presented that have isolated the deficiencies in the theory and led to improvements including the self-consistent approach where the theory is coupled with single chain Monte Carlo simulations. Using recent simulation results, we outline the strengths and weaknesses of the theory at different levels of detail, from coarse grained bead-spring models to explicit atom models. We conclude with an overview of future directions that are beginning to be undertaken. [ABSTRACT FROM AUTHOR]

Some theoretical concepts on polymer + solvent systems and Monte Carlo simulations of corresponding coarse-grained models are briefly reviewed. While the phase diagram of polymers in bad solvents invoking the incompressibility approximation for the polymer solution has been a standard problem of polymer science for a long time, a more complete understanding of compressible polymer solutions, where liquid-liquid phase separation and liquid-vapor transitions compete, has emerged only recently. After giving a phenomenological introduction, we outline and compare three complementary approaches: self-consistent field theory, thermodynamic perturbation theory and grandcanonical Monte Carlo simulation. In order to give a specific example, we focus on the mixture of hexadecane with carbon dioxide. Attention is paid to correlate the description of the phase diagram with the properties of interfaces and the nucleation barrier that needs to be overcome to form a droplet (or bubble, respectively) of critical size, necessary for the decay of the corresponding super-saturated metastable state. Particular emphasis is given to new techniques used for the computer simulation of such phase diagrams where several order parameters compete, and to systematic difficulties that still hamper the prediction of accurate nucleation barriers from the observation of droplets (or bubbles, respectively) in finite volumes. The extent to which real materials can be modeled will also be examined. [ABSTRACT FROM AUTHOR]

This review has summarized the information available in the literature regarding various copolymers that involved some aspect of controlled/living radical polymerization (CRP) for their synthesis. The topics addressed included the formation of statistical, periodic and gradient copolymers, block and graft copolymers, including tethered copolymers, and segmented polymers with star-type chain architectures. There are numerous examples of the synthesis of all of these types of copolymers using only CRP techniques, whether it is the use of nitroxide-mediated systems, ATRP, or the degenerative transfer approach. In addition, other polymerization methods have been combined with CRP to afford segmented copolymers whose compositional combinations were never before possible. Living carbocationic, carbanionic, various types of ring-opening polymerization, coordination polymerization, and even step-growth processes have all been used in conjunction with CRP methods to produce polymers with novel structures. Finally, one of the most unique approaches to the preparation of segmented copolymers has been the development of initiators and monomers that can be used for two types of polymerization processes, whether it be in a stepwise or concurrent manner. These types of advances lend themselves to the development of routes for the preparation of materials with properties that can be specifically tuned for particular applications. [ABSTRACT FROM AUTHOR]

There are three general methods for preparing graft copolymers: grafting onto, grafting through, and grafting from. Grafting onto requires the presence of complimentary functionalities on the graft unit and the backbone. Grafting through utilizes macromonomers, which are polymer chains that contain a copolymerizable moiety at the chain end. Homoor copolymerization with another monomer produces the graft copolymer. The grafting from technique employs a backbone containing reactive sites that are capable of initiating a polymerization. Each method suffers from its own particular disadvantages, but steric hindrance of the reactive center is common to all the graft copolymer routes, affecting the grafting efficiency. Figure 38 illustrates the three approaches. The least utilized in CRP is perhaps the grafting onto approach, while grafting through and grafting from are common. These will be detailed below. [ABSTRACT FROM AUTHOR]

There have been numerous literature reports on the preparation of block copolymers using CRP methods. These copolymers range from those synthesized wholly by CRP to those that involve either transformation from other living polymerization techniques (anionic, cationic, ring-opening, etc.) to CRP, or functionalization of a macromolecule that can then be used as a macroinitiator for CRP. Each of these methods will be addressed separately. Nitroxides were predominantly used for styrene containing copolymers, whereas ATRP was successful for the acrylates and methacrylates as well [42]. [ABSTRACT FROM AUTHOR]

The simplest type of copolymer is one where two or more comonomers are simultaneously copolymerized. This technique is commonly used to modify and/or improve the mechanical and physical properties of many polymers. The relative rates of incorporation of each monomer in a given set of comonomers will be dependent on their reactivity ratios. These have been determined for the radical polymerization of numerous monomer pairs [5]. Choosing comonomers that have reactivity ratios close to 1 produces statistical copolymers, where the radical chain ends react as often with their own monomer as they do with the others, assuming equal concentrations of both monomers. If both the reactivity ratios are significantly higher than 1, indicating that the radical would prefer to homopropagate rather than cross-propagate, the backbone copolymer exhibits a more blocky structure. If both are much lower than 1, indicating that both radicals would prefer to cross-propagate, the copolymer takes on an alternating structure. This is the most typical case for a radical polymerization. If the copolymerization obeys Bernoullian statistics, a random copolymer is formed. However, polymerization may follow other types of statistics (e.g., Markovian), resulting in a variety of statistical copolymers. [ABSTRACT FROM AUTHOR]

This review is focused on controlled/living radical polymerization methods for the preparation of various copolymers. A brief introduction to the subject of radical polymerization, and early attempts to control it, is followed by a detailed examination of the literature on controlled/living radical copolymerizations from the mid-1990s until 2001. The topics covered include statistical/gradient, block, graft, and star copolymers, and the polymerization methods used to produce them. These copolymers were prepared using three major controlled radical methods (either nitroxide mediated polymerization, atom transfer radical polymerization, or degenerative transfer) and a combination of polymerization techniques, including transformation chemistry or the simultaneous/dual living polymerizations, to achieve the desired chain architecture or topology. An evaluation of the current state of the field is also presented. [ABSTRACT FROM AUTHOR]

Nanometer-sized mixed metal oxide (MMO) particles (Zn1-x)MgxO) with very precise stoichiometry are prepared employing a polymer-based method. The precursor is formed by loading a polyacrylate with metal ions followed by purification of the polymer metal ion complex via repeated precipitation/redissolution cycles. Calcination of the polymer precursor at 550 °C gives particles of the metastable solid solution of the ZnO/MgO system in the composition range (× < 0.2 and × ⩾ 0.82). The MMO crystal particles are typically 20-50 nm in diameter. Doping of the ZnO by Mg2+ causes a shrinkage of lattice parameter c. Effects of band gap engineering on the optical band gap are reported. The photoluminescence in the visible is also affected, and its maximum shifts from 2.12 eV (pure ZnO) to 2.32 eV at × = 0.21. The crystalline MMO particles start to undergo segregation into hexagonal and cubic phases upon annealing at 800 °C. [ABSTRACT FROM AUTHOR]

With the combination of molecular scale information from electron paramagnetic resonance (EPR) spectroscopy and meso-/macroscopic information from various other characterization techniques, the formation of mesoglobules of thermoresponsive dendronized polymers is explained. Apparent differences in the EPR spectra in dependence of the heating rate, the chemical nature of the dendritic substructure of the polymer, and the concentration are interpreted to be caused by the formation of a dense polymeric layer at the periphery of the mesoglobule. This skin barrier is formed in a narrow temperature range of ∼4 K above TC and prohibits the release of molecules that are incorporated in the polymer aggregate. In large mesoglobules, formed at low heating rates and at high polymer concentrations, a considerable amount of water is entrapped that microphase-separates from the collapsed polymer chains at high temperatures. This results in the aggregates possessing an aqueous core and a corona consisting of collapsed polymer chains. A fast heating rate, a low polymer concentration, and hydrophobic subunits in the dendritic polymer side chains make the entrapment of water less favorable and lead to a higher degree of vitrification. This may bear consequences for the design and use of thermoresponsive polymeric systems in the fast growing field of drug delivery. [ABSTRACT FROM AUTHOR]

Abstract: We report time-resolved photoluminescence (PL) spectroscopy study of solutions and thin films of pure penta-fluorenes with varying alkyl side-chain structures. While no low-energy green emissions were observed in solutions, spin-cast and vacuum dried initial films exhibited low-energy emission in time-delayed PL spectra, centered at 490nm, whose intensity varied strongly with the side-chain structure. In comparison, the films annealed at 170°C in air showed nearly identical low-energy emission in time-delayed PL spectra, centered at 520nm, regardless of the side-chain structures. Therefore, our experimental results obtained for pure penta-fluorenes provided clear evidence that the mechanism of the green emission of oligofluorene and polyfluorene films includes both the intermolecular aggregate/excimer formation, emitted around 490nm, and fluorenone-type chemical defects, emitted around 520nm. [Copyright &y& Elsevier]